Molecular hybridization of cellular RNA with unique sequences of DNA is being employed as a measure of gene expression in several model systems in which modifications of nervous system-related functions are manipulable by environmental modification or disease states. As a follow-up to previously measured differences in RNA diversity in brains of rats subjected to different post-weaning environments, a detailed investigation was conducted of the diversity in total RNA, nuclear RNA, and poly(A)-containing mRNA of normal rat brain. From these data, measurements of RNA sequence compartmentation in brain were made, and it was determined that the differences in transcription between brains of rats raised in enriched and impoverished environments lay in the diverse RNA species which were present in lowest abundance. Further studies with the same three classes of RNA from a neuroblastoma cell line have shown that little change in RNA sequence diversity occurs as a correlate of morphological and biochemical differentiation. RNA sequence diversities of normal and dystrophic (dy2J/dy2J) mouse tissues were not distinguishable.